It is known that a salt of a fatty acid ester of hydroxycarboxylic acid such as a salt of a fatty acid ester of lactic acid is useful as an anionic surface active agent for food, cosmetics, detergents, etc. (JP-A-64-6237 and JP-A-4-23900 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")).
Representative processes for the preparation of the aforementioned compound reported so far include a process of subjecting lactic acid and a fatty acid to direct esterification at a temperature of from 100.degree. C. to 250.degree. C. in the presence of an alkali catalyst (U.S. Pat. No. 2,733,252), a process of reacting lactic acid with an acid chloride of a fatty acid (U.S. Pat. No. 2,789,992), etc. The fatty acid ester of lactic acid obtained by such a reaction is normally subjected to alkali neutralization in the form of a reaction mixture to form a salt.
However, the reaction product obtained by the aforementioned processes has a considerable amount of unreacted lactic acid or a fatty acid derived from the starting materials in addition the desired fatty acid ester of lactic acid. Further, by-products such as polylactic acids and fatty acid esters thereof are formed in a large amount. When the aforementioned mixture is subjected to neutralization with an alkali, the fatty acid ester of lactic acid is partly hydrolyzed. As a result, the final product has an extremely low purity. The acid chloride process gives a somewhat enhanced reaction yield of the desired product. However, the cost of the facility required in this process is high due to corrosion problems and thus this process is not necessarily desirable from the industrial standpoint of view.
In the past, no reports have been made with respect to the process of subjecting a salt of hydroxycarboxylic acid and a fatty acid ester of lower alcohol to effect ester interchange in an organic solvent to obtain the corresponding salt of fatty acid ester of hydroxycarboxylic acid.